Frequently, modern automotive ignition systems have controlled an ignition coil current by modulating a control terminal, e.g., a gate, of a switching device, e.g., an insulated-gate bipolar transistor (IGBT), which provides a current path for a primary winding of an ignition coil. In such automotive ignition systems, it has generally been desirable for the current in the primary winding of the ignition coil to increase as quickly as possible, limited by an impedance of the primary winding, to a predetermined desired coil current limit level. Further, when the coil current limit level has been reached, it has generally been desirable for the coil current to smoothly transition to a steady-state value, with minimal oscillation during the transition.
In a typical automotive ignition system, when an IGBT is used as the switching device, a designed current of approximately 500 uA has been used to quickly charge a gate capacitance of the IGBT and raise an IGBT gate voltage above a turn-on gate threshold of the IGBT. However, when the IGBT gate capacitance is charged to a maximum voltage level, as typically determined by an ignition control integrated circuit, the gate voltage can be maintained with significantly less current than the current initially required to quickly charge the IGBT gate capacitance. After the gate is fully charged, the lower IGBT gate current requirement continues while the primary winding current is increasing to the desired coil current limit level. When the coil current limit level is reached, the IGBT gate voltage is reduced in an attempt to maintain a constant primary winding current.
In a typical automotive ignition system, the decrease in the IGBT gate voltage has been achieved through the use of a closed-loop feedback circuit, i.e., a gate control current limit circuit. When the IGBT gate voltage is reduced, a gate drive current source that is providing the IGBT gate charging current has generally increased its output current due to changes in the current source bias conditions. Thus, in order to reduce the IGBT gate voltage, the gate control current limit circuit has been required to sink the additional current. As mentioned above, in operation, the input current draw is at a maximum during the initial charging of the IGBT gate and subsequently reduces during the time that the primary winding current is increasing to the current limit level. Finally, the current draw again increases when the gate control current limit circuit reduces the IGBT gate voltage to control the primary winding current.
In input-powered automotive ignition systems, a supply current is provided from an associated control unit, through a series resistor that is either internal or external to the ignition control integrated circuit. Unfortunately, the fluctuation in the supply current provided by the control unit, through the series resistor, causes a proportional voltage fluctuation to the gate drive current source. This voltage fluctuation, under some input conditions, causes the gate control current limit circuit to repeatedly change from an open-loop condition (IGBT fully on) to a closed-loop condition (IGBT gate voltage controlled). This voltage oscillation, in turn, causes an undesired oscillation in the primary winding current.
What is needed is a drive current stabilization circuit that substantially maintains a constant current output from a current source that is driving a control terminal of a switching device that controls a primary winding current of an ignition coil, irrespective of the state of a current limit control loop.